This invention relates to electrostatography, and more particularly to improved electrostatographic developing materials and the use thereof.
Electrostatography is best exemplified by electrophotography. The basic electrophotographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting latent electrostatic image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the latent electrostatic image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing steps.
Final copies of the toner image are generally prepared by heating the toner image on a suitable support to a temperature at which the toner flows in order to effect fusing of the toner to the support medium. In order to increase the speed at which toners may be fixed to a support attempts have been made to form toners of low molecular weight resins which are easily heat fused at relatively low temperatures, but such attempts have not generally been successful in that such toners tend to block at low temperatures.
It has been proposed that pressure-fixable toners comprising a wax in combination with a polymer material be used as toner material. Examples of these materials are those disclosed in British Pat. No. 1,210,665 and U.S. Pat. No. 3,925,219. These toners have not proved to be entirely satisfactory as they exhibit tendencies toward blocking, shortness of developer life due to impacting on the carrier, filming of the photoreceptor surface, and difficulty in attrition of the soft materials to toner size particles.
It also has been proposed to pressure-fix relatively hard polymeric material by the use of high pressures. This also is found to not be totally satisfactory as the pressures required to pressure fix conventional toners tended to degrade the strength and appearance of paper and further did not result in satisfactory fix to the paper when flexed.
It also has been proposed to utilize in pressure-fixing applications, encapsulated toners comprising a hard polymeric shell material surrounding a core of liquid ink or soft solid flowable material. Results using such toners have proved to be less than fully satisfactory in many cases as there is difficulty in forming perfect shell materials and therefore blocking of the material occurs due to leakage of the core. Further, the formation process of encapsulated material are difficult and expensive. Accordingly, there remains a need for new toners which may be pressure-fixed rapidly.